The present invention relates to power steering systems for automotive vehicles in general, and more particularly to improvements in hydrostatic power steering systems of the type wherein a valve member or spool of the hydraulic control valve is normally held in neutral position by two centering springs.
It is already known to install in a hydrostatic power steering system a pilot pump which can be actuated by the steering wheel and a main pump which is driven by a motor or generator to draw oil or another hydraulic fluid from a reservoir. Pressurized fluid can enter the one or the other chamber of a power cylinder (whose piston is connected with the steering linkage) when the spool assumes a certain position out of neutral. The spool can be shifted from neutral position to an operative position in which one of the chambers of the power cylinder receives pressurized fluid or to an operative position in which the other chamber of the power cylinder receives fluid if the pilot pump is actuated by rotating the steering wheel in the one or the other direction. When one of the chambers in the power cylinder receives pressurized fluid from the main pump, the spool connects the other chamber with the reservoir, or vice versa.
In a presently known hydrostatic power steering system of the just outlined character, the pilot pump is connected with both chambers of the power cylinder when the spool assumes its neutral position. If the operator of the vehicle thereupon rotates the steering wheel to initiate a change in direction of movement of the conveyance, the pilot pump causes fluid to flow against the one or the other end face of the spool whereby the latter moves axially and enables the main pump to cause pressurized fluid to flow through the pilot pump and to one of the two chambers in the power cylinder. A drawback of the just described power steering system is that the extent of angular movement of the steering wheel is not always truly representative of the changed direction of movement of the front wheels and also that shocks (e.g., due to external influences) which develop in the power steering system are likely to interfere with accurate steering of the vehicle.